1. Field of the Invention
The present invention relates to injection molding machines and, more particularly, to hydraulically actuated die closing units of injection molding machines in which the movable parts of the die closing unit are supported and guided by means of two or four tie rods.
2. Description of the Prior Art
Die closing units may be of the knee linkage type, where the hydraulic cylinder is oriented transversely to the opening and closing movement of the two halves of the injection molding die, or they may be of the multi-cylinder type, in which the movable parts of the die closing unit are directly attached to the piston rods of the hydraulic cylinders. In both cases, the movable assembly is supported and guided on tie rods. In the case of a multi-cylinder die closing unit, the tie rods also serve as the piston rods.
In the past, it has been common practice to connect the tie rods to the stationary and/or movable parts of the die closing unit by providing on the tie rods a reduced-diameter threaded end portion, with a ring shoulder at the inner end of the reduced-diameter portion. One such prior art configuration is disclosed, for example, in the German Offenlegungsschrift (Publ. Appln.) No. 1,629,707. There, the tie rods are stationary rods, connected on one extremity to a stationary die plate, while their opposite ends are similarly connected to a stationary thrust stock. Between these supports is arranged a knee-linkage-type hydraulic die closing mechanism. Each tie rod is thus axially clamped to a cooperating part, through the action of a nut engaging the thread of the reduced-diameter end portion of the tie rod. This connection must withstand the closing pressure exerted by the die closing unit against the two halves of the molding die, a pressure that might be as high as 60 tons, for certain injection molding machines. In view of the fact that this closing pressure on the die tends to axially separate the tie rod from its cooperating part, it has been found to be necessary for the clamping force between the ring shoulder of the tie rod and the face of the cooperating part to be at least equal to the closing pressure, in order to avoid a separation of these parts at the ring shoulder. Obviously, if such a separation takes place at the shoulder, the stability of the connection is greatly diminished and wear will take place at the surfaces of relative displacement between the parts. Considering the limitations to the diameter range of tie rods used for injection molding machines of the above-described type, it has been found that the prior art tie rod connection could not always guarantee uninterrupted maintenance of such an abutment contact, especially in cases where, in the closed position of the injection molding die, unequal pressures are generated in the main pressure cylinders of the die closing unit. Such unequal pressures may be the result of an asymmetry in the hydraulic controls of the die closing unit. If, as a result of such an uneven pressure distribution, the abutment faces of the tie rod shoulder and of the cooperating part become separated under the closing pressure, such separation will reflect itself in a deflection and misalignment of the part to which the tie rods are connected. This condition, repeating itself during every injection cycle of the machine as a rythmic application of stress and deflection, may lead to fatigue phenomena of the material in the critical zones of the tie rods, with eventual fatigue failure in these zones. It should be noted that, due to the diameter step at the tie rod shoulder, the bending and fatigue stress is concentrated around the immediate area of the annular shoulder.